Some of the debris from a collision may end up in an orbit with a higher apogee, perigee will necessarily still be at or below the altitude of the last collision and will be subject to some of the same low-orbit aerodynamic drag that starlink satellites experience; passes through lower altitudes will apply drag that will first drop the apogee and will then eventually cause the debris to reenter.

How can it linger in a higher orbit. Maybe some of the debris gets a kick which increases its velocity, but you need two velocity boosts to circularise the orbit, no? So I figure at worst you get an elliptical orbit which will still decay

Nope, if it goes up it will go down even faster.

Orbits are about speed. Two things colliding cannot have debris coming out at a faster velocity than either of them.